1. Field of the Invention
The present invention relates to a stable method for preparing a water soluble branched polymer capable of forming an aqueous solution which has low viscosity in spite of high molecular weight, and use thereof.
2. Description of Related Art
In the fields of coating materials and adhesives, a water soluble polymer having high molecular weight and low viscosity is not only excellent in coating performances, but also excellent in coating film performances. In the field of papermaking additives, a water soluble polymer having high molecular weight and low viscosity is excellent in antifoaming properties and is easy to use, but also excellent in a paper strength enhancing effect. Therefore, it is required to prepare a water soluble polymer capable of forming an aqueous solution which has low viscosity in spite of high molecular weight.
As a part of energy-saving measures and environmental conservation measures, a closed papermaking process, that is, a decrease in displacement has recently made rapid progress. Consequently, the amount of aluminum sulfate (sulfuric acid band), which has conventionally been used, has been limited as a matter of course. Therefore, even when a small or no amount of aluminum sulfate (sulfuric acid band) is added, there has been required to develop a chemical which can exert a sufficient paper-strength effect.
As a paper strength agent which can be used in a wide pH range regardless of whether or not a fixing agent such as aluminum sulfate (sulfuric acid band) is used together with the paper strength agent, for example, Japanese Unexamined Patent Publication (Kokai) No. 60-94697 proposes an amphoteric water soluble copolymer formed from acrylamide, α,β-unsaturated monocarboxylic acid or dicarboxylic acid, and a basic monomer such as dimethylaminoethyl methacrylate.
However, a conventional amphoteric copolymer exerts an insufficient paper strength enhancing effect and it has been required to develop a chemical capable of exerting more excellent effect.
Thus, there has been proposed that, in a (meth)acrylamide-based amphoteric copolymer, a copolymer containing a bifunctional vinyl monomer as a copolymer component is provided and the molecular weight is increased as compared with a copolymer containing no bifunctional vinyl monomer, thereby exerting an excellent paper strength enhancing effect (Japanese Unexamined Patent Publication (Kokai) No. 63-50597).
To further enhance the paper strength enhancing effect, a conventional single stage polymerization method has recently been improved. For example, there are disclosed the following polymerization methods:    (1) a double stage polymerization method in which an acrylamide-based monomer is polymerized in the presence of previously polymerized polyacrylamide (Japanese Unexamined Patent Publication (Kokai) No. 3-227485),    (2) a polymerization method in which a vinyl monomer is added dropwise in the presence of previously polymerized polyacrylamide (Japanese Unexamined Patent Publication (Kokai) No. 5-195485, Japanese Unexamined Patent Publication (Kokai) No. 11-228641 (Specification of U.S. Pat. No. 6,255,400),    (3) a polymerization method in which a first stage polymerization is initiated and, after reaching a specific reaction rate, a monomer is added dropwise (WO2002/053835), and    (4) a polymerization method in which a catalyst is further added to a prepolymer obtained by polymerization (Japanese Unexamined Patent Publication (Kokai) No. 5-287693, Japanese Unexamined Patent Publication (Kokai) No. 2000-212229 and Japanese Unexamined Patent Publication (Kokai) No. 7-97790).
However, the methods (1) and (2) have a drawback such as poor productivity because repolymerization and dropping polymerization must be conducted after previous polymerization. Any of the methods (1) to (4) were not satisfactory methods in view of crosslinking during polymerization, branching process, and control of polymer viscosity after the completion of the polymerization. According to the polymerization methods described above, there is a limitation in an increase of the molecular weight because of rapid thickening during the polymerization and also it is difficult to control molecular weight distribution of a polymer.
When the water soluble polymer is employed as a papermaking additive such as paper strength agents, it is effective to narrow the width of molecular weight distribution so as to obtain high effect in a small addition amount. However, any of the above methods (1) to (4) do not disclose a method for preparing a water soluble branched polymer with narrow molecular weight distribution.